The long-term goal of our laboratory is to understand molecular mechanisms underlying the formation and maintenance of synapses. Our model is the NMJ, a cholinergic synapse between motoneurons and muscle cells. The NMJ exhibits the high degree of subcellular specialization characteristic of chemical synapses. Despite significant progress in our understanding of anatomic structures of the NMJ, much less is known about mechanisms how various components are put together. In preliminary studies, we investigated the role of ??catenin in NMJ formation using a conditional knockout strategy to suppress its expression in skeletal muscles. The results demonstrate that muscle ??catenin deficiency caused post- and pre-synaptic defects including dislocation of phrenic nerve primary branches, enlarged AChR clusters, and impaired neurotransmission. In contrast, ?-catenin inactivation in motoneurons appeared to have little effect on NMJ morphology and transmission. These observations demonstrate a role of muscle ?-catenin in NMJ development, identifying a possible ?-catenin-dependent retrograde mechanism. To further test this hypothesis, we propose the following three aims of experiments. We will characterize NMJs in HSA-??cat-/- mice using electron microscopic techniques;investigate the mechanisms of defective neurotransmission in HSA-??cat-/- mice;and investigate mechanisms of how muscle ?-catenin regulates NMJ formation. Results of these experiments will contribute to a better understanding of mechanisms of muscle ?-catenin in regulating NMJ formation and pave the way to the identification of effectors of ?-catenin and possible retrograde signals. These studies will contribute to a better understanding of pathophysiology of the neuromuscular disorders and to developing strategies of gene therapy and of diagnostic tools for these diseases. The long-term goal of our laboratory is to understand molecular mechanisms underlying the formation and maintenance of synapses. This proposal is to study the role of muscle ??catenin in NMJ formation. Results of these experiments will contribute to a better understanding of mechanisms of muscle ?-catenin in regulating NMJ formation and pave the way to the identification of effectors of ?-catenin and possible retrograde signals. These studies will contribute to a better understanding of pathophysiology of the neuromuscular disorders and to developing strategies of gene therapy and of diagnostic tools for these diseases.